Oil from water separator

ABSTRACT

An oil from water separator has an oil disengagement chamber adapted to receive an oil and water mixture and retain it for a sufficient time in a relatively undisturbed state so that oil in the mixture floats to the top of the mixture. This results in a substantially oil free volume of water having a layer of oil derived from said oil and water mixture floating on the surface of the mixture. The oil disengagement chamber is partially separated from an effluent water chamber by an under flow baffle which ducts said substantially oil free volume of water to the effluent water chamber. The oil disengagement chamber has a low liquid level which is higher than the under flow baffle. The outflow of the substantially oil free volume of water from the effluent water chamber is limited to a rate of outflow which is a function of the head of the liquid in the effluent water chamber. The outflow is limited by a siphon which primes at a chamber high liquid level and loses prime at said chamber low liquid level, or by holes in a weir wall.

FIELD OF THE INVENTION

The present application relates to oil from water separators and, moreparticularly, such separators suitable for use in inground oraboveground installations where it is desired to prevent oil in waterconcentrations above a predetermined limit from being distributed to theenvironment in an uncontrolled fashion.

BACKGROUND

Mechanical oil from water separator systems are known. Devices/systemsare also known that provide settling in chambers separated bybaffles—refer the arrangement of FIG. 1 which shows a Prior Art AmericanPetroleum Institute (API) oil from water separator design. It consistsof a rectangular tank with two or more vertical partitions or baffles toseparate entry chamber, oil disengagement chamber and effluent waterchamber, and which is designed to run full of water.

The API oil from water separator is sized to provide low turbulenceconditions and sufficient residence time for oil globules with a minimumdiameter of 0.015 cm (150 microns) to separate from the oil/watermixture flowing though the separator.

This prior art system can be characterised as a decant-type system wherefor every input of liquid there is an output of a similar amount at thesame time, thereby affecting separation efficiency.

Attempts have been made in the prior art to control the level of theoil/water interface, for example see U.S. Pat. No. 5,147,534 (Rymal) andU.S. Pat. No. 4,031,007 (Sierra) and, more generally, see U.S. Pat. No.4,960,513 (Young), U.S. Pat. No. 4,436,630 (Anderson) and U.S. Pat. No.5,378,353 (Koch).

In all of these systems, whilst there has been a move away from a simpledecant-type approach, there is usually added a specific oil from waterseparation process beyond mere gravitational separation. Koch requires aspecific separate coalescer unit whilst U.S. Pat. No. 4,554,074(Broughton) utilises separation plates.

In many applications it would be desirable to employ a separator systemhaving the intrinsic simplicity of the API-type systems whilst achievingconsistent predetermined levels of separation of oil from water.

It is an object of the present invention to provide an inherently simpleoil from water separator system which provides consistent levels ofseparation of oil from water over a predetermined range of inflowconditions.

SUMMARY OF INVENTION

Accordingly, in one broad form of the invention there is provided an oilfrom water separator including an oil disengagement chamber adapted toreceive an oil and water mixture and retain it for a sufficient time ina relatively undisturbed state whereby oil in the mixture floats to thetop of the mixture resulting in a substantially oil free volume of waterhaving a layer of oil derived from said oil and water mixture floatingon the surface thereof; said oil disengagement chamber partiallyseparated from an effluent water chamber by an under flow baffle whichducts said substantially oil free volume of water to said effluent waterchamber; said oil from water separator characterised in that outflow ofsaid substantially oil free volume of water from said effluent waterchamber is limited by flow retarding means to a rate of outflow which isa function of the head of the liquid in said effluent water chamber.

In a further broad form of the invention there is provided an oil fromwater separator including an oil disengagement chamber adapted toreceive an oil and water mixture and retain it for an extended time in arelatively undisturbed state whereby oil in the mixture floats to thetop of the mixture resulting in a substantially oil free volume of waterhaving a layer of oil derived from said oil and water mixture floatingon the surface thereof; characterised in that outflow from said chamberis controlled in a predetermined way by flow retarding means.

In a further broad form of the invention there is provided an oil fromwater separation system including an oil disengagement chamber having aflush storage volume defined between a chamber high liquid level and achamber low liquid level; a liquid volume equivalent to said flushstorage volume caused to exit from said chamber on attainment of saidchamber high liquid level.

Preferably said flush storage volume is caused to exit by means of asiphon mechanism.

In a further broad form of the invention there is provided an oil fromwater separator including an oil disengagement chamber adapted toreceive an oil/water mixture and retain it for a sufficient time in arelatively undisturbed state whereby oil in the mixture floats to thetop of the mixture resulting in a substantially oil free volume of waterhaving a layer of oil derived from said oil and water mixture floatingon the surface thereof; characterised in that outflow from said chamberis prevented until said mixture reaches a predetermined chamber highliquid level whereupon said volume of water is caused to exit saidchamber.

In a further broad form of the invention there is provided an oil fromwater separator including an oil disengagement chamber adapted toreceive an oil/water mixture and retain it for a sufficient time in arelatively undisturbed state whereby oil in the mixture floats to thetop of the mixture resulting in a substantially oil free volume of waterhaving a layer of oil derived from said oil and water mixture floatingon the surface thereof; characterised in that outflow from said chamberis limited by flow retarding means to a predetermined function of thelevel of said oil and water mixture in said chamber.

Preferably said flow retarding means is operable only between a chamberlow liquid level and a chamber high liquid level.

In one particular preferred form said flow retarding means comprises atleast one siphon which primes at said chamber high liquid level andloses prime at said chamber low liquid level.

In an alternative preferred form said flow retarding means comprises atleast one bleed aperture or weep hole.

Preferably said at least one bleed aperture or weep hole is located atthe level of said chamber low liquid level.

More preferably said at least one bleed aperture or weep hole is sizedwith reference to expected inflow of said oil and water mixture intosaid oil disengagement chamber such that, during operation, the level ofsaid oil and water mixture will rise from said chamber low liquid levelup to a higher liquid level and then return to said chamber low liquidlevel, thereby defining for each situation an oil and water mixtureactive lag capacity or accumulation capacity between said chamber lowliquid level and said higher liquid level.

More preferably said active lag capacity or accumulation capacity has acharacteristic which is a function of

(a) inflow rate

(b) desired residence time of said oil and water mixture in said oildisengagement chamber.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings wherein:

FIG. 1 illustrates a Prior Art (API) separator and

FIG. 2 illustrates a separator system according to a first embodiment ofthe system.

FIG. 3 illustrates the sequence of filling and emptying of the separatorsystem of FIG. 2.

FIG. 4A is a graph of head versus flow for the separator system of FIG.2,

FIG. 4B illustrates in cross section the first embodiment system of FIG.2 to which FIG. 4A is applicable.

FIG. 5A is a graph of head versus flow for the system, of FIG. 5B,

FIG. 5B illustrates in cross section a separator system according to asecond embodiment of the invention,

FIG. 6A is a graph of head versus flow for the system of FIG. 6B,

FIG. 6B illustrates, in cross section, a separator system according to athird embodiment of the invention involving multiple weep holes,

FIG. 7 is a graph of the behaviour of water level in the system of FIG.2 in the form of a graph of water level versus time,

FIG. 8 illustrates the behaviour of the system of FIG. 2 underalternative operating conditions in the form of a graph of water levelversus time,

FIG. 9 illustrates the behaviour of the system of FIG. 5 in the form ofa graph of water level versus time,

FIG. 10 illustrates particular flow characteristics of particularimplementations of the invention (example 2) and

FIG. 11 is a top view and side section view of a separator systemaccording to a further embodiment of the invention.

FIG. 12 is a side section view of multiple separator systems connectedin a flow-through, series configuration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The Prior Art separator 10 of FIG. 1 comprises an entry chamber 11separated by a baffle 12 from an oil disengagement chamber 13 which, inturn, is separated from an effluent water chamber (15) by a baffle (14).

Various embodiments of the invention as to be described below arecharacterised in their most broad form by the addition of a flowretarding device to an outlet portion of a separator. The separator canbe in the box form of the prior art API separator of FIG. 1 or can takean alternative form (for example refer the cylinder form of example 3 ofFIG. 11 to be described later in this specification).

The flow retarding device acts to ensure that for the majority ofoperating conditions likely to be encountered, water in the storagevolume will have a sufficient residence time and flow in a sufficientlyundisturbed manner to ensure oil from water separation substantially toa predetermined value.

In the embodiments described below the flow retarding device operatescontinuously to retard flow. Embodiments differ in how the outflow ispermitted.

In all cases, accumulation occurs in the oil disengagement chamber as aresult of control of outflow.

Furthermore, it imposes an outflow rate from the separator which is afunction of the liquid head over the outflow level in the separator.

First Embodiment

With reference to FIG. 2 an oil from water separator system 20 accordingto a first embodiment of the invention is illustrated.

FIG. 3 shows a series of operating conditions A-E for the separator ofFIG. 2.

The system 20 directs an influent of oily water through or under abaffle 12 to an oil disengagement chamber 21 the water from which passesbeneath a skimmer wall or second baffle 14 to a siphon pipe 22 in an endwall 16. This siphon pipe discharges effluent water into exit pipe 25through draw off chamber 23.

The siphon pipe 22, in operation, causes the level of liquid in oildisengagement chamber 21 to move between high level 27 and low level 28.

The volume of liquid defined between these two levels forms anaccumulation capacity which is designated the flush storage volume oroil and water accumulation volume 29.

In use water laden with oil enters oil disengagement chamber 21 as inFIG. 3 with the level in the chamber 21 rising until the maximumaccumulation volume 29 is achieved at which time siphon pipe 22 operatesto cause the flush storage volume or accumulation volume 29 to exit viaexit pipe 25 until the siphon breaks at low level 28. Low level 28 isselected to be, for design conditions, such that accumulated, separatedoil cannot pass under the baffle 14 and escape from the separator oildisengagement chamber.

As more oil laden water enters oil disengagement chamber 21 the processrepeats itself in accordance with FIG. 3 C, D, E.

In this manner a relatively large volume of oil/water mixture isretained for a relatively long period of time to allow oil separation tooccur prior to siphoned exit.

Restated in other terms: A feature of this embodiment is theincorporation of one or more automatic siphons which release water onlyperiodically from an oil disengagement chamber and which chamber createsa potential storage for a selected volume of first flush oil/watermixture or a major oil spillage of a volume equal to the flush storagevolume or accumulation volume 29.

This volume 29 is sized to contain a major oil spillage or to be filledprogressively with oil/water mixture from successive rainfall events.Until this volume 29 is accumulated, oil globules can coalesce andseparate from the water over a period greater than the residence timeavailable in the standard flow through decant separator of FIG. 1 for agiven separator tank volume. The oil disengagement chamber 21 isquiescent with virtually zero turbulence except at the end of each cyclewhen the siphon is operating.

When the water surface reaches a selected chamber high liquid level 27 asiphon which discharges into draw off chamber 23 is primed wherebysubstantially oil-free water is released until the water surface fallsto a selected chamber low liquid level 28 at which the siphon breaks.This releases a volume of effluent water equal to the accumulationvolume 29 leaving capacity for the next cycle of oil/water inflows.

One can more specifically differentiate the volumes of liquid in theseparator and, more specifically in the oil disengagement chamber asfollows:

A. The flush storage volume or oil and water accumulation volume 29 aspreviously defined comprising that volume of liquid which can beaccumulated in the disengagement chamber 21 between low level 28 andhigh level 27.

B. A separated oil volume 24 defined as the volume of liquid which canbe stored in the chamber 21 between low level 28 and the lower edge 17of baffle 14 defined at under pass level 18 in FIG. 2.

C. A quiescent volume 19 defined between under pass level 18 and thebottom of the disengagement chamber 21.

As will be appreciated the quiescent volume 19 will, in use, alwayscontain a liquid. In a correctly sized and designed separator thisliquid will be substantially effluent water.

As will be further appreciated periodic flushing of the separator byoperation of the flow retarding device 26 will result in a volume ofliquid equal to the oil and water accumulation volume 29 being movedfrom the oil disengagement chamber 21 through the effluent water chamber85 and, via the flow retarding device 26 to the draw off chamber 23 andexit pipe 25. The liquid actually moved will include liquid found in allof the defined volumes 19, 24, 29, but not all of it in any oneinstance.

It is the oil and water accumulation volume 29 with its dynamic naturein that separation can take place within this volume whilst the liquidactually contained within the volume changes in quantity over time whichprovides the substantive separation characteristic and permits effectiveresidence times of the order of hours (thereby achieving effectiveoil/water separation) for a treatment capacity in a given separator sizegreater than can be achieved with an equivalent sized API-typeseparator.

It will be further observed that when outflow does occur the rate ofoutflow is a function of the head of the liquid in the effluent waterchamber 85.

FIG. 4A illustrates a head versus flow characteristic for the siphonarrangement of the first embodiment of FIG. 2.

FIG. 4B is a side section view of the siphon-based retarding device 26of FIG. 2.

Second Embodiment

FIG. 5A illustrates a second embodiment of the invention (in crosssection) comprising a flow retarding device 30 in the end wall of astorage volume 31. In this instance the flow retarding device 30comprises a retention wall 32 having a bleed aperture 33 (also termed aweep hole) therewithin which will permit the gradual release of liquidin storage volume 31 above a predetermined low level 34. The head versusflow characteristics for this arrangement are shown in FIG. 5B.

Third Embodiment

An alternative arrangement of the system of the invention according to athird embodiment is illustrated in cross section in FIG. 6A andcomprises, in this instance, a retention wall 42 in an end wall ofstorage volume 41 having within it a first bleed aperture 43, a secondbleed aperture 44 and a third bleed aperture 45 located at respectivepredetermined levels 46, 47, 48.

FIG. 6A shows a graph of head versus flow for this multiple weep holeembodiment of the flow retarding device 40.

Broadly it will be observed that the first embodiment of FIG. 2 utilisesa siphon to achieve controlled flow retardation whilst the second andthird embodiments utilise weep holes.

Whereas water will not start to flow through a siphon until a priminglevel is reached and will continue to flow until the water surfacereaches some lower level, water will flow through a hole whenever thehole is submerged on and only on the upstream side.

The objective of controlling the release of water from an oil from waterseparator is to provide residence time in the separator during which thedesired separation of oil droplets from the water can occur.

The siphon achieves this residence time by storing incoming water untilthe provided capacity is full, when the relatively oil-free water isreleased and the cycle starts again.

In some applications of a disengagement chamber for oil from waterseparation, the load may be regular as in daily washdowns and in theseapplications a slow drawdown overnight may be more desirable than thesiphon characteristic.

Such an alternative characteristic can be achieved by replacing thesiphon with weep holes, varying their number, sizes and locations toachieve any desired outflow/level relationship. This allows the watersurface in the separator to return slowly to the bottom operating levelwithout first reaching some top operating level but after a sufficienttime for oil from water separation.

The relationship between separator water level and outflow for a siphonand one or more weep holes is illustrated in FIGS. 4A, 5A and 6A asearlier described.

Relative Inflow—Outflow Behaviour

The movement in separator water level during an inflow event, however,will be broadly similar for the siphon and the weep holes, at least asfar as achieved residence time is concerned. With some generality it canbe asserted that:

An effective separator design will not require a cycle time (from risingabove the bottom operating level to returning to it) of more than 12-24hours

For rainfall runoff typical of a 1 in 1 year event, the separator canfill to the top operating level in less than an hour

The initial rise of the separator water level will be steep comparedwith the exponential fall after the outflow through the weep holes orthe siphon (see FIGS. 7, 8 and 9)

The earlier release of water through a weep hole than will occur with asiphon not yet at its priming level will have negligible effect on theinitial rise in water level

During water level fall from the top operating level, the flow throughboth the weep hole and the siphon will decline exponentially as afunction of head above the outlet

If the inflow event is not large enough to prime the siphon, the waterwill remain in the separator until there is sufficient water; with aweep hole, the water outflow will continue to decline exponentiallyuntil the weep hole level is reached, still providing (by design) thedesired residence time.

Fourth Embodiment

FIG. 11 illustrates an alternative storage volume arrangement which, asseen in plan view, takes the form of a doughnut-shaped tank 50 withinflow to a central distributor in the form of a stand pipe 51.

Outflow is from a circular retention wall 52. Controlled outflow isachieved either via a siphon pipe 53 to clarified water outlet 54 or viableed apertures (not shown) in retention wall 52 or other flow retardingmeans.

For this embodiment dimensions of the siphon pipe and/or the bleedapertures can be as for either example 1 or example 2 below.

Active Lag Capacity

With reference to FIGS. 7, 8 and 9 the previously described embodimentscan be seen to incorporate an active lag capacity or accumulation volume60 which operates above a predefined liquid low level 61 and can extendas high as a predefined liquid high level 62 set by an overflow weir(such as weir 87 in FIG. 2).

The active lag capacity 60 comes into operation when inflow to the oildisengagement chamber is such that the liquid level rises above liquidlow level 61.

Liquid low level 61 has associated with it, in these examples, eitherthe lower end of a siphon or the lowest of at least one weep hole sizedin the manner previously described and which, in combination with theend wall 16 or retention walls 32, 42, 52, forms a flow retarding meanswhich is the dominant factor which controls the shape and characteristicof the active lag capacity 60 for a given inflow characteristic andstorage volume characteristic.

The active lag capacity 60 by virtue of its coming into existence whilstthere is mismatched relative inflow and outflow from the oildisengagement chamber has a dynamic or active characteristic whichassists in efficient oil from water separation such that, for apredefined range of inflows, outflow will contain a proportion of oil inwater substantially below a predefined limit.

Interconnected Separator Units

With reference to FIG. 12 three separator units are connected in serieswhereby a first separator 81 having a lag capacity in the form of afirst active lag volume 91 feeds its output, as illustrated, directlyinto second separator unit 82 having a second active lag volume 92,which separator unit in turn feeds its outflow into third separator unit83 having a third active volume 93. In this instance the active lagcapacity of the total system is determined by the compositecharacteristic of the active lag volumes 91, 92, 93.

This arrangement has particular advantage where site shape and/or sizedictates that one large tank is inappropriate. The arrangement alsoprovides additional flexibility in terms of total residence time.

It has one particular distinguishing characteristic as compared with thesingle tank implementations in that overflow from first separator 81 inthe event of unforseen catastrophic inflow merely results in overflow ofuntreated or insufficiently treated oil/water mix into second volume 92of second separator 82 rather than the immediate discharge of untreatedor insufficiently treated oil/water mixture from the entire treatmentsystem. This multiple tank arrangement, therefore, provides a“soft-fail” mode as well as providing additional design flexibility.

Examples of the various embodiments will now be given:

EXAMPLE 1

An API type rectangular tank with siphon installed in the exit wall.Typical dimensions are 7 m long, 1.5 m wide and siphon operating levels1.6 m and 0.8 m above the floor. Volume=approx 17KL, about half of whichis the range between siphon operating levels. The siphon is made of 18mm OD hard drawn copper pipe and takes about 10 hours to draw the waterlevel down.

EXAMPLE 2

FIG. 10 illustrates a particular example of head versus flow behaviourfor the siphon embodiment of FIG. 2, the single weep hole embodiment ofFIG. 5 and the multiple weep hole embodiment of FIG. 6 for various holediameters as indicated.

The above describes only some embodiments of the present invention andmodifications obvious to those skilled in the art can be made theretowithout departing from the scope and spirit of the present invention.

It is expected that, in many embodiments, operation of the oil fromwater separator system would be unattended and/or automatic.

INDUSTRIAL APPLICABILITY

The oil from water separator device can be applied in situations such astransformer substations and other industrial sites where retention andcontrolled discharge of an oil and water mix to a specified level ofseparation is required.

What is claimed is:
 1. An oil from water separator comprising: an oildisengagement chamber adapted to receive an oil and water mixture andretain it for a sufficient time in a relatively undisturbed statewhereby oil in the mixture floats to the top of the mixture resulting ina substantially oil free volume of water having a layer of oil derivedfrom said oil and water mixture floating on the surface thereof; aneffluent water chamber partially separated from said oil disengagementchamber by an under flow baffle which ducts said substantially oil freevolume of water to said effluent water chamber, the oil disengagementchamber having a chamber low liquid level which is higher than theunderside of the under flow baffle, the outflow of said substantiallyoil free volume of water from said effluent water chamber being limitedat a rate as a function of the head of the liquid in said effluent waterchamber; and a flow retarding means comprising at least one siphoncoupled to said effluent water chamber to limit the rate of outflow ofsaid substantially oil free volume of water from said effluent waterchamber wherein said flow retarding means primes at a chamber highliquid level and loses prime at said chamber low liquid level such thatduring operation, the level of said oil and water mixture will rise fromsaid chamber low liquid level up to a higher liquid level and thenreturn to said chamber low liquid level, thereby defining an oil andwater mixture active lag capacity within an oil and water mixtureaccumulation volume in said oil disengagement chamber and wherein saidflow retarding means operates to accumulate said oil and water mixturein said oil disengagement chamber in said oil and water mixtureaccumulation volume above said chamber low liquid level and wherein saidaccumulation volume is sized with reference to inflow rate and desiredresidence time of said oil and water mixture in said oil disengagementchamber such that, for a predefined range of inflows into said oildisengagement chamber, outflow from said effluent water chamber willcontain a proportion of oil in water substantially below a predefinedlimit.
 2. The separator of claim 1 operable whereby said desiredresidence time is such that said oil and water mixture is retained insaid oil and water mixture accumulation volume in said oil disengagementchamber for an effective residence time comprising a period of time longrelative to conventional liquid full separators thereby to allow oilseparation to occur prior to periodic siphoned exit.
 3. The separator ofclaim 2 wherein said effective residence time is of the order of hours.4. The separator of claim 1 operable such that periodic flushing of saidseparator by operation of said flow retarding means will result in avolume of liquid equal to said oil and water accumulation volume beingmoved periodically from said oil disengagement chamber through saideffluent water chamber so as to exit via said flow retarding means. 5.The separator of claim 1 whereby said flow-retarding means operates toprovide an outflow characteristic of outflow from said oil and watermixture accumulation volume which has a different characteristic from aninflow characteristic of inflow into said oil and water mixtureaccumulation volume.
 6. The separator of claim 5 wherein said outflowcharacteristic is a discontinuous function of the liquid level in saideffluent water chamber.
 7. The separator of claim 5 wherein there existsa mismatch whereby said inflow characteristic is mismatched relative tosaid outflow characteristic.
 8. The separator of any one of claim 1 orclaims 2 to 7 wherein separation can take place within said oil andwater accumulation volume whilst liquid contained in said volume changesin quantity and level over time.
 9. A method of converting a separatorwhich has an oil disengagement chamber and an effluent water chamberpartially separated from said oil disengagement chamber by an under flowbaffle such that the separator normally operates liquid full, to aseparator which operates in a manner whereby liquid level in saidseparator will vary with time and rate of inflow into said separator,said method comprising: installing a flow retarding device in the formof at least one siphon in or in association with a weir wall of theseparator so that a rate of outflow of a substantially oil free volumeof water from the effluent water chamber is controlled as a function ofa head of liquid in the effluent water chamber, such that duringoperation, the oil disengagement chamber receives an oil and watermixture and retains it for a sufficient time in a relatively undisturbedstate to allow oil in the mixture to float to the top of the mixtureresulting in a substantially oil free volume of water with a layer ofseparated oil floating thereon wherein the substantially oil free volumeof water flows to the effluent water chamber under the under flow baffleto define the head of liquid in the effluent water chamber, the oildisengagement chamber having a chamber low liquid level which is higherthan the underside of the under flow baffle, and wherein the flowretarding device operates to accumulate the oil and water mixture in theoil disengagement chamber such that the level of the oil and watermixture rises from said chamber low liquid level up to a higher liquidlevel and then returns to said chamber low liquid level by the action ofthe flow retarding device thereby defining an oil and water mixtureactive lag capacity within an oil and water mixture accumulation volumein the oil disengagement chamber, wherein the flow retarding deviceprimes at a chamber high liquid level and loses prime at the chamber lowliquid level and wherein said accumulation volume is sized withreference to an inflow rate and desired residence time of said oil andwater mixture in said oil disengagement chamber such that, for apredefined range of inflows into said oil disengagement chamber, outflowfrom said effluent water chamber will contain a proportion of oil inwater substantially below a predefined limit.
 10. The method of claim 9wherein said separator after conversion according to said method isoperable whereby said desired residence time is such that said oil andwater mixture is retained in said oil and water mixture accumulationvolume in said oil disengagement chamber for an effective residence timecomprising a period of time long relative to conventional liquid fullseparators thereby to allow oil separation to occur prior to periodicsiphoned exit.
 11. The method of claim 10 wherein said effectiveresidence time is of the order of hours.
 12. The method of claim 9wherein said separator is operable such that periodic flushing of saidseparator by operation of said flow retarding means will result in avolume of liquid equal to said oil and water accumulation volume beingmoved periodically from said oil disengagement chamber through saideffluent water chamber so as to exit via said flow retarding means. 13.The method of claim 9 wherein said flow-retarding means operates toprovide an outflow characteristic of outflow from said oil and watermixture accumulation volume which has a different characteristic from aninflow characteristic of inflow into said oil and water mixtureaccumulation volume.
 14. The method of claim 13 wherein said outflowcharacteristic is a discontinuous function of the liquid level in saideffluent water chamber.
 15. The method of claim 13 wherein there existsa mismatch whereby said inflow characteristic is mismatched relative tosaid outflow characteristic.
 16. The method of any one of claim 9 orclaims 10 to 15 wherein separation can take place within said oil andwater accumulation volume whilst liquid contained in said volume changesin quantity and level over time.
 17. An oil from water separation systemcomprising: a plurality of oil from water separators wherein each oilfrom water separator comprises: an oil disengagement chamber adapted toreceive an oil and water mixture and retain it for a sufficient time ina relatively undisturbed state whereby oil in the mixture floats to thetop of the mixture resulting in a substantially oil free volume of waterhaving a layer of oil derived from said oil and water mixture floatingon the surface thereof; an effluent water chamber partially separatedfrom said oil disengagement chamber by an under flow baffle which ductssaid substantially oil free volume of water to said effluent waterchamber, the oil disengagement chamber having a chamber low liquid levelwhich is higher than the under flow baffle, the outflow of saidsubstantially oil free volume of water from said effluent water chamberbeing limited at a rate as a function of the head of the liquid in saideffluent water chamber; a flow retarding means comprising at least onesiphon coupled to said effluent water chamber to limit the rate ofoutflow of said substantially oil free volume of water from saideffluent water chamber wherein said flow retarding means primes at achamber high liquid level and loses prime at said chamber low liquidlevel such that during operation, the level of said oil and watermixture will rise from said chamber low liquid level up to a higherliquid level and then return to said chamber low liquid level, therebydefining an oil and water mixture active lag capacity within anaccumulation volume in said oil disengagement chamber and wherein saidflow retarding means operates to accumulate said oil and water mixturein said oil disengagement chamber in said accumulation volume above saidchamber low liquid level and wherein said accumulation volume is sizedwith reference to inflow rate and desired residence time of said oil andwater mixture in said oil disengagement chamber such that, for apredefined range of inflows into said oil disengagement chamber, outflowfrom said effluent water chamber will contain a proportion of oil inwater substantially below a predefined limit; and wherein said pluralityof separators are connected in series whereby outflow from eachpreceding separator passes to an inlet of the next succeeding separator.18. The separation system of claim 17 wherein each said separator isoperable whereby said desired residence time is such that said oil andwater mixture is retained in said oil and water mixture accumulationvolume in said oil disengagement chamber for an effective residence timecomprising a period of time long relative to conventional liquid fullseparators thereby to allow oil separation to occur prior to periodicsiphoned exit.
 19. The separation system of claim 18 wherein saideffective residence time is of the order of hours.
 20. The separationsystem of claim 17 wherein each said separator is operable such thatperiodic flushing of said separator by operation of said flow retardingmeans will result in a volume of liquid equal to said oil and wateraccumulation volume being moved periodically from said oil disengagementchamber through said effluent water chamber so as to exit via said flowretarding means.
 21. The separation system of claim 17 wherein saidflow-retarding means operates to provide an outflow characteristic ofoutflow from said oil and water mixture accumulation volume which has adifferent characteristic from an inflow characteristic of inflow intosaid oil and water mixture accumulation volume.
 22. The separationsystem of claim 21 wherein said outflow characteristic is adiscontinuous function of the liquid level in said effluent waterchamber.
 23. The separation system of claim 21 wherein there exists amismatch whereby said inflow characteristic is mismatched relative tosaid outflow characteristic.
 24. The separation system of any one ofclaim 17 or claims 18 to 23 wherein separation can take place withinsaid oil and water accumulation volume whilst liquid contained in saidvolume changes in quantity and level over time.